Choosing Op Amps

How important is slew rate? Shown at top is an oscillator's 1kHz square wave output. The bottom wave shows the already obvious "vertical" distortion from a slow 2V/us op amp; in this case, a '70s-era RC-4136.

There are no black-and-white answers to upgrade questions. Even identical circuits behave differently, depending on the board layout and components used. To paraphrase an old saying, learn to fish and you'll answer your own questions. I encourage experimentation, but you really need an oscilloscope to keep an eye on things.

For simplicity, op amps can be distilled into three application-specific categories: high-gain preamp, general-purpose buffer and line driver (output amp). Op amps aren't supposed to have color; the output signal should be the same as the input signal with the least amount of additional artifacts. All op amps are designed to have low distortion, low noise and wide bandwidth; however, some turn out better than others.

PACKAGING

For our purposes, IC op amps typically come in two packages: 8-pin DIP (dual inline pin) for single and dual op amps and 14-pin DIP for quad op amps. All are easily socketed. Many chips are also available in the surface-mount (SMT) package. Because the pin-outs are pretty much standardized, the interchangeability factor is a tempting tease that implies plug-and-play upgrade potential, but comes with the warning, “Proceed with caution.” With the Internet providing easy access to data sheets and a deluge of opinions, the only question left is, “How to choose?”

The specs are mind-numbing at first. Supply voltage range is self-explanatory, but with names like open-loop gain, bandwidth, distortion, noise, slew rate, DC offset and settling time, some specs are tangible and some are not. Gain is a variable that affects each of these parameters and stability. Increased gain and signal levels raise distortion and awareness of noise while decreasing bandwidth. What starts out as several megahertz might be reduced to the upper kilohertz when the gain is 100 times the original signal.

TAMING THE COWBOYS

In this age of kissing digital zeroes for fear of losing resolution, one short-term fix for what might seem like “bad analog sound” is to back off, take advantage of headroom and let noise be your dither. Seriously, no amplifier sounds its best when pushed toward the rails except when you are specifically looking for distortion. Op amps are not internally designed to emulate guitar amps.

It is nearly impossible to resist the temptation to compare specs. Yes, a modern op amp boasting a slew rate of 20-volts-per-microsecond (V/µs) is better than 30-year-old 2V/µs technology, but that doesn't imply that 2,000 V/µs is going to be as noticeable an improvement. Such demon-speed may be a gift to those in search of “effortless sonics,” but taking advantage of video-agile ICs is not a plug-and-play upgrade. That's the domain of people with RF skills.

There are plenty of vintage consoles out there limping along at 2 V/µs that just happen to sound amazing for that old-fashioned rock 'n' roll sound. You might prioritize your upgrade investigation by targeting the preamp ICs or, if outboard preamps are plentiful, focus on the mix bus and master module as Dan Kennedy did for his Trident 65 upgrade (featured in “Tech's Files,” December 2000).

DESIGNER CIRCUITRY

Despite the ease of ICs, some designers still choose discrete circuitry. Rather than initially obsessing over specs, their goal is simply to realize an idea in hardware. Each person has tests that weed out problems and assist in maintaining high standards — a type of headroom that translates to reliable performance in the field under adverse conditions. And yes, many designers do not have engineering degrees. This should be encouraging to all you “tweakers” out there. Now get to work! You've got some catching up to do.

When I asked Crane Song's Dave Hill for an example, his discrete output amplifier had to pass an acid test of driving a 100kHz square wave into a 75-ohm load. If it still looks like a square wave and remains stable, then the circuit is a winner. In this case, a square wave reveals an amplifier's ability to drive a capacitive load (a potential cause for instability), which is what 1,000 feet of cable looks like to a piece of audio gear.

Kennedy is using discrete amplifiers in the AC signal path and ICs in the DC servo loops for his new EQ-2NV, a vintage Neve-inspired equalizer. DC offset and DC stability (settling time) are two issues that can reach a critical mass in high-gain applications (for example, in a mic preamp). Such idiosyncrasies reveal themselves in funny ways — as scratchy pots and switches — as do oscillations.

DC offsets are measured in millivolts (mV) and hopefully microvolts (µV), a rather intangible specification compared to the brute-force transistor stages of the original single-ended, Class-A Neve designs. The output amps are biased to operate around 12 volts (half of the power supply), so blocking caps are used throughout. If a capacitor becomes leaky (develops a parallel resistance), then DC from one stage can re-bias another stage to the point of compromising headroom, or worse, creating nasty distortion. Similarly, a transient pumped through a Neve amplifier stage will typically bump all of the bias points. With an oscilloscope, you can easily watch the recovery, a snail's crawl compared to the needs of digital audio converters that rely on amplifiers with fast settling times (on the order of a few hundred nanoseconds).

At the other end of the spectrum, Jim Williams of Audio Upgrades typically chooses video op amps for audio applications. It is a different philosophy aimed at improving existing equipment and not recommended for your first D.I.Y. I recently repaired a mic preamp in a Fostex portable DAT recorder, substituting an OPA2604 for an NE5532. Some might view this as an upgrade, but I saw it as using what's on-hand. This is a somewhat less-romantic notion than what some would imagine; designers know what works, what doesn't and where the laws of diminishing returns come into play. You should know that the OPA2604 was totally unstable without additional de-coupling — a challenge in a cramped surface-mount village.

I've put together a chart of some of the most popular IC op amps available at www.mixonline.com. Happy reading!